1. Field of the Invention
The present invention relates to displacement coders which, in robots, automata, machining centers and other automatic machines, are associated with the different mobile members with a view to locating the exact position of each of them in the sequence of displacements.
2. History of the Related Art
Between the absolute direct-reading coders and the incremental coders having relative count-down, intermediate systems are known in which the precise reading of the positioning is effected by a so-called operation of calibration which consists in displacing the mobile member in order to find the position thereof by counting pulses.
U.S. Pats. Nos. 4,363,026 to SALMON and 4,041,484 to GROFF and British Pat. No. 1,565,400 400 to OTIS ELEVATOR CO., for example, propose displacement coders comprising a first disc angularly connected to the displacement of the member in question, a second disc rotatably connected to the first in a ratio close to 1 but different from this value, and pulse counting means associated with each of the discs for revealing the angular shift existing therebetween.
In this way, precise data are obtained at a cost much lower than that of the absolute coders, by means of a prior operation of calibration of much reduced amplitude, not capable of being hindered by the surroundings of the mobile member to be verified. However, the dimensions of these coders which may be qualified as "differential" are large and hinder installation thereof, and their structure is complicated.